1. Field of the Invention
The present invention relates to an optical communication system, more especially to an optical communication system with use of a light-transmitting medium, such as a plastic optical fiber and the like.
2. Description Related to the Prior Art
In a light-transmitting medium (such as an optical fiber, an optical lightguide, an optical lens and the like), plastic materials are more excellent than a quarts materials, usually in points of workability for molding, weight saving of parts, lower cost, flexibility, impact resistance and the like. However, for example, it is usually not preferable to use a plastic optical fiber (POF) for an optical transmission for a long distance, since the transmission loss of the light is larger in the POF than the quartz optical fiber. On account of the characteristics of plastics, a diameter of a core of the POF can be enlarged in a low cost, so as to be equal to or more than 10 μm. Further, in the enlargement of the diameter, it becomes unnecessary to more precisely connect an optical fiber to several peripheral parts and devices to be used for branching or connecting the optical fibers. Therefore, the POF has merits of the easiness of connecting the peripheral parts and devices, the easiness for the end processing, the unnecessity of alignment, and the like. Furthermore, because of the characteristics of plastics, the POF has merits of not only the decrease of the cost at the connector, but also a low dangerousness of sticking into a human body, an easiness of processing that is caused by the high flexibility, an easiness of laying, a resistance to vibration and the like. Thus the POF attracts attentions for using not only in home or car, but also for an interconnection in a high speed data processor, a extremely short cable with high capacity in a DVI (digital Video Interface) and the like.
Recently, instead of the quartz optical fiber with which the communication of the wavelength at 780 nm, 850 nm or the like can be made, Japanese Patent Laid-Open Publications No. 5-224033 (see Page 3) and 8-5848 (see Pages 3–7) suppose a POF containing fluorine, a POF in which deuterium atoms is substituted for hydrogen atoms, and the like, with which the transmission loss is low even in the wavelength range of near-infrared ray.
The POF is constructed of a core and a clad. Usually, the core is a central part made of organic compounds in which the polymers form a matrix. The clad is an outer part having a lower refractivity than the core.
As a production method of the POF, there are a method in which the polymer is extracted in a fiber-shape to form a core and a clad simultaneously, and a method in which a preform is produced and thereafter drawn with heating in an atmosphere from 180° C. to 250° C.
Several methods of forming the preform are proposed. As a forming a clad portion of the preform, there is a melt-extrusion method of the polymer, a method in which the monomers (methylmethacrylate and the like) is supplied in a case having a resiliency and the case is rotated to make the polymerization of the monomers.
In the tube-shaped clad portion, a core portion having the refractive index distribution is formed. As a method of providing the refractive index distribution for the core portion, the Japanese Patent Laid-Open Publication No. 2-16504 (see, Pages 2–3) teaches a method in which two or more sorts of polymers having different refractive indices or different ratio of refractive index changes overlay to form a multi-layer material, and extruded in cocentrical arrangement to form the core portion. Further, in the Japanese Patent Laid-Open Publications No. 5-181023 (Pages 3–6) & 6-194530 (Pages 3–7) teach a method in which are mixed polymerization initiators and monomers forming a different polymer from that of the clad portion. In this method, while the mixture is dripped into an inner space in the clad portion, the mixture is heated to make the polymerization. Furthermore, in a method described in a brochure of International Publication under PCT No. 03/08488, after a mixture of a monomers, a refractive index modifier and polymerization initiator is filled in the clad portion, the polymerization is made in the heating to form the core portion, and concentration distribution of refractive index modifier contained in the core portion provides the refractive index distribution. Further, in the method in the Japanese Patent Laid-Open Publications No. 6-297596, the mixture ratio of the polymers having different refractive indexes is continuously varied.
However, the polymer (such as PMMA, PC or the like) of the light-transmitting medium as the above POF is absorbable of water, and when the POF absorbs water, the transmission loss becomes larger, and larger in the wave length range of the near-infrared ray. Since the water absorbability of the polymer cannot be made zero, the optical communication with use of the POF become unstable in the near-infrared ray wavelength range.